feat: @agentaos/pay SDK - regulated stablecoin payments

[PancheI]

Summary

New SDK package for accepting regulated stablecoin payments: @agentaos/pay

SDK (packages/pay/):

• checkouts.create() - standalone or from payment link template
• checkouts.list(), .retrieve(), .cancel()
• paymentLinks.create(), .list(), .retrieve(), .cancel()
• transactions.list() - unified inbound/outbound ledger
• invoices.list(), .retrieve(), .void(), .downloadPdf(), .downloadStatement(), .exportCsv()
• webhooks.verify() - HMAC-SHA256 signature verification
• Zero dependencies, ESM only, Node.js 20+

Example (examples/pay-express-shop/):

• Complete Express e-commerce integration
• Checkout creation, webhook verification, order fulfillment, polling fallback

Usage

import { AgentaOS } from '@agentaos/pay';

const agentaos = new AgentaOS('sk_live_...');

const checkout = await agentaos.checkouts.create({
  amount: 49.99,
  currency: 'EUR',
  successUrl: 'https://shop.com/success',
  webhookUrl: 'https://shop.com/webhooks',
});

console.log(checkout.checkoutUrl);

Platform PR

Companion platform changes: https://github.com/AgentaOS/platform/pull/6

Test plan

• npm install @agentaos/pay resolves
• checkouts.create() returns checkoutUrl
• webhooks.verify() correctly rejects invalid signatures
• Express example runs end-to-end
• Browser detection throws error
• Invalid API key format throws error